1. Field of the Invention
Solutions containing a base (hereinafter referred to as a base-containing solution) are formed in many industrial processes. For example, sodium hydroxide-containing waste solutions are formed in the digestion of pulp in the viscose rayon industry, base-containing solutions are also formed when treating metals and in the recovery of ion-exchange resins. It is necessary to conduct selective concentration and recovery of the base from the base-containing solution or dealkalization of the base-containing solution to produce a highly concentrated base solution. Alternatively, the solution can be further processed to produce a useful product or subjected to further treatment to prevent water pollution when it is discharged.
2. Description of the Prior Art
It has been proposed to selectively recover the base in a concentrating compartment adjacent to a diluting compartment by feeding the base-containing solution into the iondiluting compartment of an electrodialytic cell as one method of concentrating or removing the base from the-base-containing solution. However, in conventional electrodialysis, the base, such as hydroxyl ions, leaks through the cation-exchange membrane by diffusion caused by the concentration difference or electrophoresis. When the concentration of the base in the concentrating compartment is low, this phenomena causes little difficulty. However, when the concentration of the base in the concentrating compartment is high, the efficiency of the electric current is greatly decreased and industrial operation is not feasible.
It has been known to produce aqueous base-containing solutions, such as aqueous alkali hydroxide solutions by electrolytic dialysis of an aqueous alkali chloride solution in the diaphragm method using a cation-exchange membrane as a diaphragm, because the process creates no pollution. In the conventional process, cation-exchange membranes have been used as partitions between the anodes and cathodes to form anolyte compartments and catholyte compartments. An aqueous alkali chloride solution is fed to the anolyte compartments and the flow of electric current produces an aqueous alkali hydroxide solution in the catholyte compartment by electrolytic dialysis. In this process, when the concentration of the alkali hydroxide base in the catholyte compartment is increased, hydroxyl ions leak through the cation-exchange membrane by the diffusion caused by the concentration difference and electrophoresis. Accordingly, when the concentration of alkali hydroxide in the catholyte compartment is low, this phenomena causes little difficulty. However, when the concentration of the alkali hydroxide in the catholyte compartments has an industrial concentration equal to or greater than 15 wt.%, the current efficiency is less than 70% and industrial operation is seldom attempted. Accordingly, there exists a need for a process for the electrodialysing aqueous base solutions wherein diffusion of the base does not occur.